Wires, such as used in the windings of transformers and motors, have been insulated with enamels which are applied from organic solvent, and their application typically involved health, fire and air pollution hazards. To avoid these problems, wire could instead be insulated by kraft paper, but such a process is slow and expensive and undesirably increases the bulk of the wire winding.
To overcome these deficiencies, epoxy resin powders have been designed to provide coatings which exhibit good electrical insulating properties without pollution of the atmosphere. One such powder having found excellent commercial acceptance is that disclosed in U.S. Pat. No. 4,267,300.
This patent is based on an epoxy resin powder which contains a high molecular weight epoxy resin and trimellitic anhydride as the curing agent. It has been found that the anhydride curative provides an excellent combination of stability, adhesion, cure rate, electrostatic properties for the powder, and excellent electrical properties to the cured coating. However, the anhydride has two properties which, if improved, could lead to an even more effective powder for wire coatings. Since trimellitic anhydride is high melting, extreme care is necessary to insure uniform dispersion thereof in the powder. Furthermore, since the anhydride is present in discrete particulate form, problems relating to the homogeneity of the powder can occur, often resulting in the formation of pinholes and resulting low dielectric strength. Also, trimellitic anhydride is relatively toxic, and therefore problems can occur in the manufacture of the powder.
I have now discovered that these deficiencies can be overcome to a large extent by the use of an adduct which is resinous and fusible and which results in high dielectric strength coatings while retaining the foregoing desired physical, chemical and electrical properties of the powder and the resultant cured coating therefrom.
Adducts of trimellitic anhydride have been disclosed in the art to serve as curatives for epoxy resins.
For example, tetracarboxylic acid hardeners were prepared from trimellitic anhydride and dihydric alcohols in U.S. Pat. No. 3,269,975. Adducts based on trimellitic anhydride and polyols having 3 or 4 (or more) hydroxyl groups are disclosed in U.S. Pat. No. 3,555,111. Similarly, another based on trimellitic anhydride and long chain diols is disclosed in U.S. Pat. No. 3,639,345.
All of the aforementioned adducts were apparently developed to impart enhanced flexibility to the cured epoxy resins. They are, however, all based on aliphatic alcohols and polyols, thus failing to impart desired high temperature stability available through use of the present adducts. Additionally, the ring and/or aromatic structure of the present adducts provides improved resin characteristics, important for formulating powder systems.